The present invention relates, in general, to growing single crystal ingots and, more particularly, to growing large single crystal ingots.
A method for producing single crystal ingots of semiconductor materials is by a technique called the Czochralski method. Using the Czochralski method to grow a single crystal silicon ingot includes forming a melt of polycrystalline silicon in a crucible heated by resistance heating or high frequency heating. A single crystal seed is attached to a seed holder that is connected to an end of a pulling shaft or a cable and dipped into the melt of polycrystalline silicon. Subsequently, the crystal is pulled from the melt while rotating at a specified speed. The diameter of the crystal can be controlled by adjusting the temperature of the melt and/or by adjusting the rate at which the crystal is pulled from the melt. Generally, the temperature of the melt is altered by changing the power provided to the heater.
A well known problem in growing a single crystal ingot is the formation of dislocations in the single crystal ingot. This problem is overcome by beginning the crystal pull relatively quickly to form a thin neck having a diameter ranging from approximately 3 millimeters (mm) to approximately 6 mm. This seeding method is typically referred to as the Dash technique. Once a zero dislocation crystal neck has been achieved, the pulling rate is reduced, resulting in the growth of a larger diameter body from the thin neck. This is a conventional method for growing single crystal ingots having diameters less than 200 mm.
Currently, the semiconductor industry is interested in larger diameter ingots. Further, it is more time efficient and cost efficient to manufacture a given number of semiconductor wafers by growing and slicing a small number of long ingots than by growing and slicing a large number of short ingots. A problem is that when a large ingot is being grown, the thin neck is subjected to more stress than it can structurally handle. Two stresses affect the thin neck, tensile stress from the growing weight of the ingot, and torsional stress from the rotational viscosity drag force of the solid-liquid interface. Both stresses increase with an increase in the diameter of the ingot. The tensile stress also increases with an increase in the length of the ingot. If these combined stresses become greater than the yield strength of the thin neck, the thin neck may break, or more commonly, may generate dislocations in the single crystal.
Accordingly, it would be advantageous to have a single crystal ingot and a method and an apparatus for growing the single crystal ingot. It is desirable for the single crystal ingot to have a large diameter and a long length. It is also desirable for the method and the apparatus to remedy the foregoing and other deficiencies inherent in the prior art when growing a large single crystal ingot.